NICHOLSON 9KIDDAW GRAPTOLITES. 



131 



species of Tetragrapsus occur in the Slates, all well-known Cana- 

 dian forms, viz. T. hryonoides, Hall, T. quadrlhrachiatus, Hall, 

 and T. Headi, Hall ; but several other species are found in the 

 Quebec group, and may subsequently be recognized in Britain. 



4. Tetragrapsus Headi, Hall. 



GraptoUthus Headi, Hall (Grapt. Quebec group, p. 94, pi. 5. 

 figs. 11, 12, pi. 6. fig. 8). 



A single specimen, apparently belonging to this species, has been 

 found at Barff, and is now in the possession of Prof. Harkness. 

 As the cellules are not exhibited, it is somewhat uncertain whether 

 the form is T. Headi, Hall, or T, crucifer, Hall ; but the shape of the 

 disk more closely resembles that of a small specimen of the former. 

 The frond consists of four stipes arising from a short funicle, the 

 divisions of which are embraced by a corneous disk. The diameter 

 of the disk is about ^V ^^ ^^ ^^^^ from side to side ; and its form is 

 four-sided, oblong, or somewhat diamond-shaped, owing to its 

 being prolonged for a short distance along the stipes. The stipes 

 are narrow at their origin, but widen out to about -^ of an inch in 

 breadth. The cellules are not displayed at all; but according to 

 Hall they are about 24 in an inch, elongate, and with submucronate 

 denticles. 



This unique specimen is the only one yet found in the Skiddaw 

 Slates in which the central disk is retained. 



Loc. Barff, near Keswick. 



5. Tetragrapsus quadribrachiatus, Hall, sp. 



Tetragrapus crucialis, Salt. (Quart. Journ. Soc. vol. xix. p. 138). 

 GraptoUthus quadribrachiatus Hall (Grapt. Quebec group, 

 p. 91, pi. 5. figs. 1-5, pi. 6. figs. 5, 6). 



This rare species would be more appropriately designated by the 

 specific name of '^ crucialis^ ^ than by that of ^' quadrlhrachiatus -^''^ 

 but the date of the latter is earlier. The frond consists of four 

 simple monoprionidian stipes, given off by a funicle of variable 

 length, two from each extremity. The stipes diverge widely from 

 their origin, but are symmetrical, unless distorted by pressure; and, 

 when well preserved, the stipes arising from opposite ends of the 

 funicle, and from its opposite sides, are parallel with one another, 

 so as to form a figure like the letter X. The stipes are narrow at 

 their origin, never attaining a great width ; and the cellules are 

 from 22 to 24 in an inch. The denticles are acute, but are not 

 produced or mucronate as in T. hryonoides, Hall ; and no disk has 

 ever been discovered to be present in this species. 



Loc. Outerside, near Keswick. 



6. Tetragrapsus brtonoides, Hall, sp. 



GraptoUthus hryonoides, Hall. (Grapt. Quebec group, p. 84, 



pi. 4. figs. 1-11). 

 Didymograpsus caduceus. Salt. (Quart. Journ. Geol. Soc. vcl. 



xix. p. 137, fig. 13 a 6). 



VOL. IXIV. PART I. L 



